That alone doesn't always fix the problem. I 've encountered the same problem many times, on the same project, and I have checked Silverlight debugging.

What I found was that the code was not always compiling, or atleast not copied to the appropriate directory. We use TFS as our source control system, and TFS marks every file as Read only, which sometimes poses a problem for silverlight builds. I had to check out the XAP file and then it usually worked, but still no guarantees :(

just another point: when attaching to a running process, the type of code needs to be "Silverlight"

new learners often keep this as "managed code"

If you want to debug your javascript code under Silverlight you have to uncheck the 'Silverlight' checkbox. You cannot debug both at the same time according to msdn.

That checkbox is saved in the "csproj.user" file in the web site project that hosts the Silverlight application, so that file must be writable and have the property:

<WebProjectProperties>

...

  <SilverlightDebugging>True</SilverlightDebugging>

...

</WebProjectProperties>

When working on teams where the .user files are not in source control (and rightfully not), every time a fresh copy is retrieved from source control that user will not have silverlight debugging enabled and their breakpoints will not hit because the .user file will not exist yet.

Also, merely setting the checkbox and saving the project does not necessarily enable debugging either (possibly because it does not cause a rebuild of the xap in clientbin)..

I know that in the web project properties - Silverlight Applications tab, you can remove and re-add the silverlight project which enables debugging properly.